Petroleum oils have been conventionally used as lubricating oils in internal combustion engines. In the past, a thinner, lighter weight oil had to be used in colder climates in order to provide sufficient fluidity for initial lubrication at low temperatures. However, as the engine continued to operate and heat up the oil, the oil became thinner and at higher operating temperatures had insufficient viscosity for optimum lubrication. In warm weather operation a heavier weight oil was required, because the thinner, lighter weight oil provided insufficient lubrication at such higher temperatures. The viscosity-temperature relationship of an oil is expressed as its "viscosity index".
In order to improve the viscosity index of lubricating oils, it has been proposed to incorporate various polymeric materials into the base stock in order to improve the inherent viscosity-temperature characteristics of the lubricating oil. One common class of commercial polymeric viscosity index improvers are the methacrylate polymers, such as polymethacrylate esters. High molecular weight viscosity modifiers formed from alpha-olefin polymers have been proposed, such as those having a weight average molecular weight in excess of 50,000, as described in U.S. Pat. No. 3,795,616 to Heilman et al.
Lower molecular weight alpha-olefin oligomers and copolymers have been proposed for use as base fluids, such as those produced, and thereafter hydrogenated, using boron trifluoride and a co-catalyst, such as n-butanol described in U.S. Pat. No. 4,032,591 to Cupples et al. However, such hydrogenated oligomers have had little or no effect when attempts have been made to utilize small amounts of such materials as viscosity index improvers for lubricating oils.
The aforementioned U.S. Pat. No. 3,795,616 to Heilman et al describes polymers of alpha-olefins having from 5 to 12 carbon atoms which are useful as viscosity enhancers for lubricating oils. The alpha-olefin polymers of Heilman et al are high molecular weight materials having a weight average molecular weight between 50,000 and 1,000,000 and a ratio of weight average to number average molecular weight of from 1 to 12. Heilman et al also teach that any Ziegler-Natta type catalyst can be employed to prepare these polymers.
U.S. Pat. No. 3,346,498 to DeVries describes high molecular weight copolymers of alpha-olefins and diolefins having 11 to 15 carbon atoms which lower the pour point of waxy mineral lubricating oils. These polymers are taught as having a molecular weight of 50,000 to 1,000,000 and are prepared using a catalyst containing a titanium halide and an organo aluminum compound.
Canadian Patent No. 734,980 to Sauer et al discloses lower molecular weight polymers of alpha-olefins having 6 to 16 carbon atoms which are useful as synthetic lubricating oils having a high viscosity index. The polymers of Sauer et al are described as having an average molecular weight ranging from about 300 to 2,000. These polymers are prepared using a catalyst of titanium tetrachloride and an organo aluminum compound, wherein the molar ratio of titanium to aluminum is from 2:1 to 20:1. Sauer et al teach that this molar ratio is important to produce normally liquid polymers. If the molar ratio of titanium to aluminum is below 2:1, Sauer et al teach that undesirable solid polymers are produced.
U.S. Pat. No. 3,403,197 to Seelbach et al discloses low molecular weight unsaturated polymers of alpha-olefins having from 3 to 40 carbon atoms. The polymers of Seelbach et al are described as having a cryoscopic molecular weight of from 150 to 1,500 and are prepared using a catalyst consisting of violet titanium trichloride and a monoalkyl aluminum dihalide.